Bovine ephemeral fever virus in cell culture and mice

Light, immunofluorescent and electron microscopic observations were carried out sequentially on mice and VERO cell cultures infected with bovine ephemeral fever (BEF) virus. In early harvests from cell culture, 185×73 nm cone-shaped particles with nearly parallel sides predominated; these particles had all other features typical of the Rhabdoviruses (surface projections, envelope, axial channel, precisely coiled helical nucleocapsid with 35 cross-striations at a 4.8 nm interval). Identical particles were the most frequent form in mouse brain. Variation in shape toward more broadly based cone-shapes occurred late in infection and reflected anomalous morphogenesis or particle breakdown. T (truncated) particles were common at all stages of infection. It was concluded that the reported variation in length and shape of particles in various BEF virus isolates is not necessarily releated to "strain variation" but more likely to varying growth rates and T particle interference associated with varying degrees of adaptation to a host system. BEF viral morphogenesis took place primarily upon plasma membranes in vivo and in cell culture in association with small accumulations of intracytoplasmic matrix. Fusion of viral envelopes was observed, and an early syncytium formation occurred in infected cell cultures. Cytopathology was similar in vivo and in cell culture; a protracted stage of cell rounding was followed by extreme cytoplasmic vacuolation and condensation and then by lysis. Necrotic encephalomyelitis was severe in moribund mice but extraneural sites of viral propagation and damage were not found with any of the techniques employed. Since this strict neurotropism is most likely not characteristic of naturally acquired BEF virus infection in nature, it was concluded that the mouse was an unsatisfactory host for the further study of pathogenesis.